For a variety of diagnostic and related reasons, it is important to be able to accurately determine lung volume. Such measurements are crucial in evaluating lung damage as a result of disease or trauma. The measurements are also important in analyzing the extent to which blood is accommodated in the lungs during breathing, for example under stress conditions.
Accurately determining an individual's lung volume is a key parameter in pulmonary physiology and diagnosis but one that is not easily measured, as it involves various parameters and cannot be readily measured based the volume of air exhaled. The difficulty in measuring lung volume measurements stems from the fact that the lungs do not fully collapse. Lung physiology and the mechanical properties of the lungs and chest wall, including the ribs, leave a significant amount of air in the aerated portions of the lungs, after exhaling fully.
Although it is relatively straightforward to measure the volume of air which is exhaled, at the end of complete exhalation, this is not indicative of true lung volume, as a significant amount of air is always left in the lungs. This is due to the fact that the lungs do not collapse completely, complicating lung volume measurements.
The gas left in the lungs at the end of a complete exhalation is termed the Residual Volume (RV) which may be significantly increased in disease states. The total volume of gas in the lungs at the end of a maximal inspiration is termed the Total Lung Capacity (TLC) which includes the RV plus the maximum amount of gas which can be inhaled or exhaled, which is termed the Vital Capacity (VC).
As previously indicated, during normal breathing the subject does not empty the lungs down to RV nor inflate them to TLC. The amount of gas in the lungs at the end of a normal breath is termed the Functional Residual Capacity (FRC), which is distinct from a complete exhalation. A further measurement is Total Gas Volume (TGV) of the lungs in a patient.
Various techniques have been proposed for measuring the various lung air volumes. At least two of these techniques are in common use including gas dilution and plethysmography technique utilizing a device called a body box,
The gas dilution technique makes use of a spirometer which contains a certain known concentration of a gas not normally found in the lungs, such as helium. After steady state is achieved the gas is analyzed chemically and the determined concentration of the helium is used to calculate the patient's FRC.
However the gas dilution technique requires the use of certain expensive and difficult to handle gases, such as helium and xenon. Furthermore, the technique requires the use of a gas analyzer. Finally, it is not normally possible to use the technique to measure lung capacity under stress since the measurement typically takes from 3 to 7 minutes which is ordinarily longer than the time of the stress.
During plethysmography a patient is placed in a body box which is hermetically sealed and utilized to measure the TGV. While in the sealed body box the patient breathes through a breathing tube. The airflow through the breathing tube is blocked at certain intervals. By blocking the airflow while in a sealed and controlled environment allows the measurement of relating the changes in pressure in the chamber to calculate the patient's TGV utilizing Boyle's law.
However, the plethysmography or body box technique requires large and expensive body box. Furthermore the device is cumbersome and is not applicable for ambulatory use, or home use requiring appropriate clinical environment and conditions. The body box does not allow for performing the measurement under stress conditions since the body box is confining and since stress would lead to a warming of the air in the body box, thereby reducing the accuracy of the measurements. Finally, the plethysmography technique requires the patient to simulate normal breathing but with a blocked breathing tube which is difficult for some people to accomplish, especially old people and young children, that further reduces the accuracy of the technique The body box further requires patient active cooperation and therefore cannot be performed on immobile individuals or individuals confined to a bed or patients in a vegetative state or comatose patients.
Body plethysmograph devices for determination of TGV are disclosed, for example, in U.S. Pat. No. 6,113,550 to Wilson, and have been known and used since at least 1955. Other devices, which include the use of impedance belts have been disclosed as well, for example, in U.S. Pat. No. 5,857,459. In both types of devices, the plethysmograph chamber or the impedance belts are designed so that the volume in the lungs can be calculated directly, so as to provide reliable measurement parameters for calculation of TGV. As indicated above these methods for measuring TGV are all less than optimal, requiring a sealed chamber in which the subject sits, or external belts which have been shown not to provide reliable results and which may be bulky, expensive and inconvenient to operate, and require full cooperation of the subject during the measurement maneuvers to obtain meaningful results
Recent developments for example as described in US Patent Publication No. US 2011/0282228 describe a desk top device that offers an alternative to the body box method for determining lung volumes utilizing a method known as partial volume method. The partial volume method utilizes short interruptions of airflow through the flow tube in order to determine the lung volume. Such desk top devices, while they are smaller and more compact than the body box, are still cumbersome and do not provide an ambulatory solution.
Other small scale devices that provide an alternative to the body box are described in U.S. Pat. No. 6,183,423 to Gaumond et al; U.S. Pat. No. 5,233,998 to Chowienczyk et al. Both devices utilize controlled short interruptions of airflow where airflow through the flow tube is occluded, in order to allow for measurements of lung volumes using inferences from Boyle's law.